Method of operating an electron beam physical vapor deposition apparatus

ABSTRACT

An electron beam physical vapor deposition (EBPVD) apparatus and a method for using the apparatus to produce a coating material (e.g., a ceramic thermal barrier coating) on an article. The EBPVD apparatus generally includes a coating chamber that is operable at elevated temperatures and subatmospheric pressures. An electron beam gun projects an electron beam into the coating chamber and onto a coating material within the chamber, causing the coating material to melt and evaporate. An article is supported within the coating chamber so that vapors of the coating material deposit on the article. The operation of the EBPVD apparatus is enhanced by the inclusion or adaptation of one or more mechanical and/or process modifications, including those necessary or beneficial when operating the apparatus at coating pressures above 0.010 mbar.

This application is a divisional of Ser. No. 09/624,809, filed on Jul.24, 2000, now abandoned.

This application claims benefit of Provisional Patent Application No.60/147,236, filed Aug. 4, 1999, which is hereby incorporated byreference.

FIELD OF THE INVENTION

This invention generally relates to an electron beam physical vapordeposition coating apparatus. More particularly, this invention isdirected to such a coating apparatus adapted to deposit ceramic coatingson components, such as thermal barrier coatings on superalloy componentsof gas turbine engines.

BACKGROUND OF THE INVENTION

Higher operating temperatures for gas turbine engines are continuouslysought in order to increase their efficiency. However, as operatingtemperatures increase, the high temperature durability of the componentsof the engine must correspondingly increase. While significant advanceshave been achieved with iron, nickel and cobalt-base superalloys, thehigh-temperature capabilities of these alloys alone are often inadequatefor components located in certain sections of a gas turbine engine, suchas the turbine, combustor and augmentor. A common solution is tothermally insulate such components in order to minimize their servicetemperatures. For this purpose, thermal barrier coatings (TBC) formed onthe exposed surfaces of high temperature components have found wide use.

To be effective, thermal barrier coatings must have low thermalconductivity and adhere well to the component surface. Various ceramicmaterials have been employed as the TBC, particularly zirconia (ZrO₂)stabilized by yttria (Y₂O₃), magnesia (MgO) or other oxides. Theseparticular materials are widely employed in the art because they can bereadily deposited by plasma spray and vapor deposition techniques. Anexample of the latter is electron beam physical vapor deposition(EBPVD), which produces a thermal barrier coating having a columnargrain structure that is able to expand with its underlying substratewithout causing damaging stresses that lead to spallation, and thereforeexhibits enhanced strain tolerance. Adhesion of the TBC to the componentis often further enhanced by the presence of a metallic bond coat, suchas a diffusion aluminide or an oxidation-resistant alloy such as MCrAlY,where M is iron, cobalt and/or nickel.

Processes for producing TBC by EBPVD generally entail preheating acomponent to an acceptable coating temperature, and then inserting thecomponent into a heated coating chamber maintained at a pressure ofabout 0.005 mbar. Higher pressures are avoided because control of theelectron beam is more difficult at pressures above about 0.005 mbar,with erratic operation being reported at coating chamber pressures above0.010 mbar. It has also been believed that the life of the electron beamgun filament would be reduced or the gun contaminated if operated atpressures above 0.005 mbar. The component is supported in proximity toan ingot of the ceramic coating material (e.g., YSZ), and an electronbeam is projected onto the ingot so as to melt the surface of the ingotand produce a vapor of the coating material that deposits onto thecomponent.

The temperature range within which EBPVD processes can be performeddepends in part on the compositions of the component and the coatingmaterial. A minimum process temperature is generally established toensure the coating material will suitably evaporate and deposit on thecomponent, while a maximum process temperature is generally establishedto avoid microstructural damage to the article. Throughout thedeposition process, the temperature within the coating chamber continuesto rise as a result of the electron beam and the presence of a moltenpool of the coating material. As a result, EBPVD coating processes areoften initiated near the targeted minimum process temperature and thenterminated when the coating chamber nears the maximum processtemperature, at which time the coating chamber is cooled and cleaned toremove coating material that has deposited on the interior walls of thecoating chamber. Advanced EBPVD apparatuses permit removal of coatedcomponents from the coating chamber and replacement with preheateduncoated components without shutting down the apparatus, so that acontinuous operation is achieved. The continuous operation of theapparatus during this time can be termed a “campaign,” with greaternumbers of components successfully coated during the campaigncorresponding to greater processing and economic efficiencies.

In view of the above, there is considerable motivation to increase thenumber of components that can be coated within a single campaign, reducethe amount of time required to introduce and remove components from thecoating chamber, and reduce the amount of time required to performmaintenance on the apparatus between campaigns. However, limitations ofthe prior art are often the result of the relatively narrow range ofacceptable coating temperatures, the complexity of moving extremely hotcomponents into and out of the coating chamber, and the difficultiesconfronted when maintaining an advanced EBPVD apparatus. Accordingly,improved EBPVD apparatuses and processes are continuously being soughtfor depositing coatings, and particularly ceramic coatings such as TBCs.

BRIEF SUMMARY OF THE INVENTION

The present invention is an electron beam physical vapor deposition(EBPVD) apparatus and a method for using the apparatus to produce acoating (e.g., a ceramic thermal barrier coating) on an article. TheEBPVD apparatus of this invention generally includes a coating chamberthat is operable at an elevated temperature (e.g., at least 800° C.) anda subatmospheric pressure (e.g., between 10⁻³ mbar and 5×10⁻² mbar). Anelectron beam gun is used to project an electron beam into the coatingchamber and onto a coating material within the chamber. The electronbeam gun is operated to melt and evaporate the coating material. Alsoincluded is a device for supporting an article within the coatingchamber so that vapors of the coating material can deposit on thearticle.

According to the present invention, the operation of the EBPVD apparatuscan be enhanced by the inclusion or adaptation of one or more featuresand/or process modifications. According to one aspect of the inventionrelating to process temperature control, the coating chamber containsradiation reflectors that can be moved within the coating chamber toincrease and decrease the amount of reflective heating that the articlereceives from the molten coating material during a coating campaign.Process pressure control is also an aspect of the invention, by whichprocessing pressures of greater than 0.010 mbar can be practiced inaccordance with copending U.S. patent application Ser. No. 09/108,201 toRigney et al. (assigned to the same assignee as the present invention)with minimal or no adverse effects on the operation and reliability ofthe electron beam gun, and with minimal fluctuations in processpressures. Mechanical and process improvements directed to this aspectof the invention include modifications to the electron beam gun, thecoating chamber, and the manner by which gases are introduced andremoved from the apparatus. Also improved by this invention is theelectron beam pattern on the coating material.

According to another preferred aspect of the invention, a crucible isemployed to support the coating material within the coating chamber. Thecrucible preferably comprising at least two members, a first of whichsurrounds and retains a molten pool of the coating material, while thesecond member is secured to the first member and surrounds an unmoltenportion of the coating material. The first and second members define anannular-shaped cooling passage therebetween that is closely adjacent themolten pool, so that efficient cooling of the crucible can be achieved,reducing the rate at which the process temperature increases within thecoating chamber.

Another preferred aspect of the invention entails a rotatable magazinethat supports multiple, ingots of the coating material beneath thecoating chamber. The magazine is indexed to individually align multiplestacks of one or more ingots with an aperture to the coating chamber forsequentially feeding the ingots into the coating chamber withoutinterrupting deposition of the coating material.

According to another preferred aspect of the invention, a viewport isprovided for viewing the molten coating material within the coatingchamber. In order to be capable of providing a view of the extremelyhigh-temperature process occurring within the coating chamber, theviewport is fluid-cooled and has a high rotational speed stroboscopicdrum and a magnetic particle seal that provides a high-temperaturevacuum seal for the stroboscopic drum. Another preferred aspect is thatthe viewport provides a stereoscopic view of the coating chamber, bywhich one or more operators can simultaneously observe the coatingchamber while retaining stereoscopic vision.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic top and front views, respectively, of anelectron beam physical vapor deposition apparatus used to deposit acoating material in accordance with this invention.

FIGS. 3, 4 and 5 are cross-sectional views taken along section line 3—3of FIG. 1, and showing a movable platform employed in accordance withone aspect of this invention.

FIGS. 6 and 7 are more detailed front and top cross-sectional views,respectively, of preferred interior components for a coating chamber ofthe apparatus of FIGS. 1 and 2.

FIGS. 8 and 9 compare an EB gun orifice of the prior art and an orificeconfigured in accordance with the preferred embodiment of thisinvention.

FIG. 10 is a cross-sectional view of a crucible housing an ingot ofcoating material and an electron beam projected onto the surfaces of thecrucible and ingot in accordance with the preferred embodiment of thisinvention.

FIG. 11 is a plan view of the crucible of FIG. 10 and a preferredpattern for the electron beam on the crucible and ingot.

FIG. 12 depicts a preferred power intensity distribution of the electronbeam pattern across the surface of the ingot and crucible of FIGS. 10and 11.

FIG. 13 shows a preferred viewport for observing the process within thecoating chamber of the apparatus shown in FIGS. 1 and 2.

FIG. 14 shows a control panel for monitoring and controlling theoperation of the apparatus of FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

An EBPVD apparatus 10 in accordance with this invention is generallydepicted in FIGS. 1 and 2, with various components and features beingdepicted in FIGS. 3 through 14. The apparatus 10 is particularly wellsuited for depositing a ceramic thermal barrier coating on a metalcomponent intended for operation within a thermally hostile environment.Notable examples of such components include the high and low pressureturbine nozzles and blades, shrouds, combustor parts and augmentorhardware of gas turbine engines. While the advantages of this inventionwill be described with reference to depositing a ceramic coating on suchcomponents, the teachings of this invention can be generally applied toa variety of coating materials and components.

For purposes of illustrating the invention, the EBPVD apparatus 10 isshown in FIGS. 1 and 2 as including a coating chamber 12, a pair ofpreheat chambers 14, and two pairs of loading chambers 16 and 18, sothat the apparatus 10 has a symmetrical configuration. The front loadingchambers 16 are shown as being aligned with their respective preheatchambers 14, with parts 20 originally loaded on a rake 22 within thelefthand chamber 16 having been transferred to the preheat chamber 14and, as depicted in FIG. 1, into the coating chamber 12. With thesymmetrical configuration of the apparatus 10, while the parts 20 loadedthrough the front lefthand loading chamber 16 are being coated withinthe coating chamber 12, a second batch of parts in the front righthandloading chamber 16 can be preheated in the righthand preheat chamber 14,a third batch of parts can be loaded into the rear lefthand loadingchamber 18, and a fourth batch of parts can be unloaded from the rearrighthand loading chamber 18. Consequently, four process stages canoccur simultaneously with the preferred EBPVD apparatus 10 of thisinvention.

According to a preferred embodiment of this invention, the loadingchambers 16 and 18 are mounted to low-profile movable platforms 24, sothat the loading chambers 16 and 18 can be selectively aligned withtheir preheat chambers 14. For example, when the front lefthand loadingchamber 16 is brought into alignment with the lefthand preheat chamber14 to allow the parts 20 to be inserted into the coating chamber 12, therear lefthand loading chamber 18 is set back from the lefthand preheatchamber 14, so that parts can be simultaneously loaded or unloaded fromthe rake 22 of the rear lefthand loading chamber 18. Each platform 24 isalso preferably movable to a maintenance position, in which neither ofits loading chambers 16 and 18 is aligned with its preheat chamber 14,so that the interiors of the preheat and loading chambers 14, 16 and 18can be accessed for cleaning. The platforms 24 are preferably supportedat least in part by roller bearings 44 mounted in the floor, though itis foreseeable that a variety of bearings could be used. Each platform24 has a low elevational profile (projection above the floor) of notmore than one inch (about 2.5 cm) with a chamfered edge (preferably 30degrees from horizontal), which together essentially eliminate thepotential for an operator tripping on the edge of the platform 24.Stationary objects surrounding the apparatus 10 are preferablypositioned away from the edges of the platforms 24 to avoid an operatorbeing pinched by a platform 24 when it is repositioned. As alternativesto the platform configuration shown, platform systems with multipleoverlapping or telescoping movable segments could be used. Furthermore,the movable segments could slip beneath a fixed elevated platformsurrounding the platform assemblies. Finally, separate preheat chamberscould be provided for the loading chambers 16 and 18, so that bothloading chambers 16 and 18 and their heating chambers would besurrounded by a movable platform system.

As shown in FIGS. 3 through 5, a portion of the coating chamber 12 isalso preferably configured to move relative to the preheat chamber 14 inorder to facilitate cleaning of the interior of the chamber 12 betweencoating campaigns. As seen in FIG. 3, the coating chamber 12 is in itsoperating position with a viewport 48, described in greater detailbelow, mounted to a front section of the chamber 12. In FIG. 4, thefront section of the coating chamber 12 (as well as an ingot magazine102 associated with the coating chamber 12 and discussed below) is shownas having been moved away from the remainder of coating chamber 12 inorder to access a movable work platform 50, which is shown rotated intoa working position in FIG. 5. In this position, the interior of thecoating chamber 12 can be easily accessed by the work platform 50. Theplatform 50 is shown as being coupled with a hinge 53 to the base of thecoating chamber 12, though it is foreseeable that other acceptablestructures could be employed. The platform 50 can be configureddifferently from that shown in FIGS. 3 through 5, including a hingedsegmented construction, and with kick plates and other safety-relatedaccessories.

The coating, preheat and loading chambers 12, 14, 16 and 18 areconnected by valves (not shown) that achieve a vacuum seal between thesechambers. To maximize the size and number of parts 20 that can be loadedbetween the chambers 12, 14, 16 and 18, the valves preferably have aminimum dimension of about 250 mm, which is considerably larger thanpreviously thought practical by those skilled in the art. Because thecoating, preheat and loading chambers 12, 14, 16 and 18 must be pumpedto varying levels of vacuum, and in some cases are required to moverelative to each other as explained above, the valves must be capable ofnumerous cycles at relatively high pressures. Seal designs suitable forthis purpose are known in the art, and therefore will not be discussedin any detail.

With reference to FIGS. 6 and 7, coating is performed within the coatingchamber 12 by melting and evaporating ingots 26 of ceramic material withelectron beams 28 produced by electron beam (EB) guns 30 and focused onthe ingots 26. Intense heating of the ceramic material by the electronbeams 28 causes the surface of each ingot 26 to melt, forming moltenceramic pools from which molecules of the ceramic material evaporate,travel upwardly, and then deposit on the surfaces of the parts 20,producing the desired ceramic coating whose thickness will depend on theduration of the coating process. While two ingots 26 are shown in theseFigures, it is within the scope of this invention that one or moreingots 26 could be present and evaporated at any given time.

EBPVD coating chambers are typically capable of being maintained at avacuum level of about 0.001 mbar (about 1×10⁻³ Torr) or less. In theprior art, a vacuum of at most 0.010 mbar, and more typically about0.005 mbar, would be drawn within the coating chamber 12 during thecoating process, the reason being that higher pressures were known tocause erratic operation of the EB guns 30 and make the electron beams 28difficult to control, with the presumption that inferior coatings wouldresult. It has also been believed that the life of the gun filamentwould be reduced or the gun contaminated if operated at coating chamberpressures above 0.005 mbar. However, in accordance with copending U.S.patent application Ser. No. 09/108,201 to Rigney et al., assigned to thesame assignee as this invention, the coating chamber 12 is preferablyoperated at higher pressures that surprisingly yield a ceramic coatingwith improved spallation and impact resistance, as well as promote thecoating deposition rate in conjunction with higher ingot evaporationrates than that achieved in the prior art.

Rough pumpdown can be performed in the coating, preheat and loadingchambers 12, 14, 16 and 18 with, mechanical pumps 31. A cryogenic pump32 of a type known in the art is shown in FIGS. 1 and 2 as beingemployed to aid in the evacuation of the coating chamber 12 prior to thedeposition process. Also shown in FIGS. 1, 3, 4 and 5 is a diffusionpump 34 whose operation is similar to those known in the art, butmodified with a throttle valve 36 to regulate the operation of the pump34 in accordance with this invention. More particularly, the throttlevalve 36 is actuated between an open position (FIG. 3) and a closedposition (FIGS. 4 and 5) as well as positions therebetween. The benefitof the throttle valve 36 is realized when the vacuum within the coatingchamber 12 is maintained at the relatively high pressures employed bythis invention. When the maximum operating capacity of the diffusionpump 34 is required to evacuate the coating chamber 12, the throttlevalve 36 is open as shown in FIG. 3. For processing hardware, thecoating chamber 12 must be maintained at the targeted pressure (e.g.,0.015 mbar), necessitating that the throttle valve 36 is moved to apreset throttled position some distance from the fully closed positionof FIGS. 4 and 5. As seen in FIG. 1, separate diffusion pumps 38similarly equipped with throttle valves (not shown) are preferablyemployed to evacuate the preheat chambers 14, again for the reason thata relatively high pressure is desired for the coating operation of thisinvention. The mechanical pumps 31 preferably include leak detectorconnections 33 to which a leak detector can be connected for detecting asystem vacuum leak using helium or another gas that can be safelyintroduced through leaks in the chambers 12, 14, 16 and 18, orassociated equipment.

With reference again to FIGS. 1 and 2, the loading chambers 16 and 18are generally elongated in shape, and are equipped with loading doors 40through which parts are loaded onto the rakes 22. The loading chambers16 and 18 are also equipped with access doors 42 to motion drives(schematically represented at 46 in FIG. 1) that control the operationof the rakes 22. More particularly, the parts 20 supported on the rakes22 are preferably rotated and/or oscillated within the coating chamber12 in order to promote the desired coating distribution around the parts20. The access doors 42 allow the operator of the apparatus 10 toquickly adjust or change the settings of the motion drives 46 withoutinterfering with loading and unloading of parts from the loadingchambers 16 and 18.

Referring again to FIGS. 6 and 7, the interior of the coating chamber 12will be described in more detail. In order to address the aforementionedproblems concerning the control of the electron beams 28 and protectionof the EB guns 30 at the higher coating pressures employed by thisinvention, certain improvements were made to the EB guns 30 and thecoating chamber 12. As seen in FIG. 6, oxygen and argon gases areintroduced into the coating chamber 12 through an inlet 54 located nearcrucibles 56 that support the ingots 26 within the coating chamber 12and retain the molten pools of ceramic material produced by the electronbeams 28. The flow rates of oxygen and argon are individually controlledbased on the targeted process pressure and the targeted partial pressureof oxygen. To reduce the occurrence of pressure oscillations within thecoating chamber 12, the control loop response time for these gases wasreduced by physically placing the control valves 58 for the gasesimmediately adjacent to the inlet 54 just outside the coating chamber12, as shown in FIGS. 1 and 6. Placement of the control valves 58 soclose to the coating chamber 12 provided a surprisingly significantimprovement in pressure control, reducing pressure fluctuations withinthe coating chamber 12 and reducing disturbances in the focus andposition of the electron beams 28 on the ingots 26.

To further improve the electron beam focus and pattern, the EB guns 30are relatively isolated from the higher coating pressure within thecoating chamber 12 by a condensate hood 52 that catches most of thesuperfluous ceramic vapors that do not deposit onto the parts 20. Thehood 52 is configured according to this invention to define a coatingregion around the parts 20, within which the elevated pressure desiredfor the coating process is specifically maintained. To facilitatecleaning between coating campaigns, the hood 52 is preferably equippedwith screens 76 that can be removed and cleaned outside of the coatingchamber 12. Preferably, the screens 76 are retained by spring pins 78instead of threaded fasteners in order to simplify removal of thescreens 76 when in the condition of having been coated with a layer ofthe coating material by the end of a campaign. Though generally morecomplicated, the entire condensate hood 52 could be removed and replacedwith a second clean hood 52.

Because the hood 52 surrounds the parts 20 as seen in FIG. 6, anaperture 62 is necessary for each beam 28 through the hood 52. Topromote the capability of maintaining higher pressures within thecondensate hood 52 as compared to the remainder of the coating chamber12, including the vicinity around the EB guns 30, the apertures 62 arepreferably formed to have dimensions of not more than that necessary toallow the electron beams 28 to pass through the hood 52. For thispurpose, the apertures 62 are preferably cut with the electron beams 28during the setup of the EBPVD apparatus 10, so that each aperture 62 hasa cross-sectional area that is approximately equal to that of itselectron beam pattern at the intersection with the hood 52.

To further isolate the EB guns 30 from the elevated pressure within thecondensate hood 52, the beams 28 travel from their respective guns 30through chambers 64 formed between the interior walls of the coatingchamber 12 and the condensate hood 52. Preferably, the diffusion pump 34has an inlet near and pneumatically coupled to each of the chambers 64.Because of the minimum size of the apertures 62, the elevated pressurewithin the condensate hood 52 (achieved by the introduction of oxygenand argon with the inlet 54) bleeds into the chambers 64 at asufficiently reduced rate to enable the diffusion pump 34 to maintainthe chambers 64 at a pressure lower than that within the condensate hood52.

FIGS. 6, 8 and 9 illustrate additional protection provided to the EBguns 30 with this invention. As is generally conventional, the EB guns30 are equipped with vacuum pumps 66 that maintain pressures within theguns 30 at levels of about 8×10⁻⁵ to about 8×10⁻⁴ mbar, which is wellbelow that existing outside the guns 30, i.e., within the EBPVD coatingchamber 12 of this invention as well as typical EBPVD coating chambersof the prior art. In order for such low pressures to be maintained, theelectron beams 28 must pass through cylindrical orifices 68 to exit theguns 30, as schematically shown in FIG. 6. FIG. 8 represents aconventional configuration for such an orifice 168. To allow for a rangeof beam focussing conditions represented by focus positions A, B and Cfor an electron beam 128 shown in FIG. 8, the orifice 168 has arelatively large diameter and length, e.g., about 30 mm and about 120mm, respectively. The disadvantage of the prior art is the reducedprotection that such a large orifice 168 can provide to the EB guns 30operating in the higher pressure environment of the apparatus 10 of thisinvention. During an investigation leading to this invention, testingevidenced that improved control of processing conditions enabled anoptimum position of the beam focus point (D in FIG. 9) to be identified.A more effective orifice design was then investigated, resulting in theorifice 68 of this invention shown in FIGS. 6 and 9, which is depictedin FIG. 9 as having a smaller diameter and length than that of the priorart orifice 168 of FIG. 8. A preferred diameter and length for theorifice 68 are believed to be about 15 and 50 mm, respectively, thoughoptimum values for these dimensions can vary depending on pressures andfocus, deflection coil current, and overall geometries.

As noted above, the condensate hood 52 is positioned around the parts 20to minimize the deposition of ceramic material on the interior walls ofthe coating chamber 12. According to this invention, the condensate hood52 is also specially configured to regulate heating of the parts 20 asrequired to maintain an appropriate part temperature during a coatingcampaign. More particularly, the hood 52 is equipped with a movablereflector plate 72 that radiates heat emitted by the molten surfaces ofthe ingots 26 back toward the parts 20. At the initial startup of acampaign, during which the temperature of the coating chamber 12 isrelatively low, the reflector plate 72 is positioned close to the parts22 with an actuator 74 to maximize heating of the parts 20. As thetemperature within the coating chamber 12 rises during an ongoingcampaign, the reflector plate 72 is moved away from the parts 20 (asshown in phantom in FIG. 6) to reduce the amount of radiated heatreflected back onto the parts 20. In this manner, the parts 20 can bemore readily brought to a suitable deposition temperature (e.g., about925° C.) at the start of a campaign, while attainment of the maximumallowed coating temperature (e.g., about 1140° C.) is delayed tomaximize the length of the coating campaign. The hood 52 plate 72 alsopromote a more uniform and stable blade coating temperature, whichpromotes the desired columnar grain structure for the ceramic coatingson the parts 20. To maintain the desired relatively high pressure withinthe condensate hood 52 while the reflector plate 72 is in the raisedposition, a water-cooled shroud 75 is shown that surrounds the plate 72to inhibit gas flow between the condensate hood 52 and plate 72, andthereby reduces pressure loss between the hood 52 and plate 72.

Shown in FIG. 7 are manipulators 77 that extend into the coating chamber12 through a ball joint feed-through 79 in the chamber wall. Themanipulators 77 are used to assist in regulating the heating of theparts 20 by moving ceramic or ceramic-coated reflectors 80 (shown as agranular material in FIG. 10) toward or away from the crucibles 56during a coating campaign. More specifically, due to their proximity tothe crucibles 56, the reflectors 80 are at a very high temperatureduring the coating process, and therefore radiate heat upward toward theparts 20. The amount of heat radiated by the reflectors 80 is generallyat a maximum when the reflectors 80 are closest to the crucibles 56, andcan be reduced by moving the reflectors 80 away from the crucibles 56.The reflectors 80 are preferably supported on a fluid-cooled plate 81that does not appreciably radiate heat to the parts 20. As a result, thereflectors 80 can be used in conjunction with the reflector plate 72 toregulate the temperature of parts 20 being coated within the coatingchamber 12 during an ongoing campaign. At the beginning of a campaign,the reflectors 80 are originally located near the crucibles 56 tomaximize heating of the parts 20, and later moved with the manipulators77 away from the crucibles 56 to reduce the amount of radiated heat.

To survive the coating chamber environment, the portions of themanipulators 77 within the coating chamber 12 are preferably formed of ahigh-temperature alloy, such as a nickel-base alloy such as X-15.Instead of a granular material, the reflectors 80 could be inessentially any form and have essentially any shape. For example, one ormore plates coated with a reflective material could be used. As a matterof convenience, the reflectors 80 could be relatively large pieces cutfrom ingots of a material similar to that being deposited, though it isapparent that other ceramic materials could be used.

As noted above, the ingots 26 of ceramic material are supported withinthe coating chamber 12 by crucibles 56 that retain the molten pools ofceramic material produced by the electron beams 28. One of the crucibles56 is shown in greater detail in FIG. 10 as having a three-piececonfiguration. An upper member 82 with a tapered upper surface 84 isassembled with a lower member 86, forming therebetween a coolant passage88 through which water or another suitable coolant is flowed to maintainthe temperature of the crucible 56 below the melting temperature of itsmaterial. A restriction plate 90 is also shown in FIG. 10, whosethickness can be selected to change, e.g., decrease, the cross-sectionalflow area of the passage 88 between a coolant inlet 92 and outlet 94.For reasons of thermal conductivity, a preferred material for thecrucible 56 is copper or a copper alloy, necessitating that the coolantflow rates through the passage 88 must be sufficient to keep thecrucible wall 96 nearest the molten portion of the ingot 26 well belowthe temperature of the molten ceramic. As is evident from FIG. 10, andas further discussed in reference to FIGS. 11 and 12, the electron beam28 is preferably projected onto the tapered surface 84 as well as theingot 26. Consequently, in order for the exterior surface of the uppermember 82 to be adequately cooled, the thickness of the wall 96 must beminimized to promote heat transfer without jeopardizing the mechanicalstrength of the crucible 56. The multiple-piece crucible configurationof this invention facilitates the fabrication of an optimalconfiguration for the coolant passage 88, as well as enables thethickness of the wall 96 to be produced with tight tolerances. While anoptimal configuration will depend on various factors, a preferredcoolant flow rate is about five to fifty gallons/minute (about twenty totwo hundred liters/minute) using water at a pressure of about two to sixatmospheres (about two to six bar) through a passage 88 whosecross-sectional area is about 400 mm², and with a maximum wall thicknessof about 10 mm adjacent the surface 84, and about 7 mm adjacent theingot 26.

FIGS. 11 and 12 represent a preferred pattern for the electron beams 28on the ingots 26 to form the pools of ceramic material. As seen in FIGS.10 and 11, the beam 28 is also projected onto that portion of thecrucible surface 84 immediately surrounding the ingot 26, with theperimeter of the beam 28 on the crucible surface 84. The preferred powerdistribution 98 of the electron beam 28 is shown in FIG. 12 as havingpeaks located near the ingot-crucible interface, with little or no poweraimed at the center of the ingot 26. According to this invention, thebenefit of directing such high beam intensities away from the center ofthe molten pool is a reduced tendency for spitting, which is generallywhen a droplet of molten ceramic is ejected from the pool duringcoating. Spitting is associated with defects in the coating produced onthe parts 20, and therefore is preferably avoided. Projecting the beam28 onto the crucible 56 serves to reduce the amount of ceramic thatmight otherwise buildup on the crucible 56 due to spitting, and alsoprovides a more even temperature distribution across the molten pool asdetermined with infrared imaging. When YSZ is used as the ingotmaterial, suitable beam intensities at the peaks in FIG. 12 are on theorder of about 0.1 kW/mm², as compared to a maximum level of about 0.01kW/mm² at the center of the pool.

Also shown in FIG. 10 is that the electron beam 28 is incident on thesurface of the ingot 26 at an oblique angle so as to establish relativeto its respective EB gun 30 a proximal intersection point 100 and anoppositely-disposed distal intersection point 101 with the crucible 56at the perimeter of the beam pattern. As shown in FIG. 11, the preferredbeam pattern intensity on the ingot 26 and crucible 56 slightlydiminishes, preferably by about 30% to 70% relative to the remainingperimeter of the beam pattern, at locations on the crucible 56corresponding to the proximal and distal intersection points 100 and101. The purpose of reducing the intensity of the beam pattern at theproximal intersection point 100 is to reduce erosion of the crucible 56by the beam 28, while reducing the beam intensity at the distalintersection point 101 has been shown to reduce waves generated by thebeam 26 on the molten ceramic pool from pushing molten ceramic over theedge of the crucible 56.

Another preferred control feature of this invention for the electronbeams 28 is the ability to temporarily interrupt the beam pattern on thesurface of the crucibles 56 with a separate higher-intensity beampattern 97 dedicated to achieving a faster evaporation rate over a smallarea in order to evaporate any ceramic that may become deposited on thecrucibles 56 as a result of spitting. This feature of the invention canbe performed during the coating operation with minimal or no impact onthe deposition process. In a preferred embodiment, when the operatorinitiates an excursion of the separate pattern 97 to evaporate a buildupof ceramic on the crucible 56, the pattern 97 is first automaticallyrepositioned to a known position, from which the pattern 97 can then bemanually moved under the direction of the operator toward the ceramicbuildup. By automatically returning the pattern 97 to a known position,the likelihood of errors that could lead to damage of the crucible 56 isreduced. Alternatively, the position of the pattern 97 could bepreprogrammed so that the operator can enter the location on thecrucible 56 onto which the pattern 97 is to be projected. Ceramicbuildup on the crucible 56 that cannot be readily removed with thepattern 97 can often be removed with the manipulator 77 shown in FIG. 7.

Magazines 102 that house and feed the ingots 26 up through the floor ofthe coating chamber 12 and into the crucibles 56 can be seen in FIGS. 1through 7. As most readily seen in FIGS. 2, 6 and 7, each magazine 102has a number of cylindrical channels 104 in which the ingots 26 areheld. The magazines 102 rotate to index ingots 26 into alignment withthe crucibles 56. The magazines 102 can also move toward and away fromeach other (i.e., laterally relative to the coating chamber 12) in orderto make adjustments for crucible separation and thereby optimize thecoating zone over which the deviation of coating thickness isacceptable. The feed mechanisms used to grip and feed the ingots 26 intothe crucibles 56 generally include clamping arms 60, each of which isdisposed at an angle from horizontal and adapted to hold the evaporatingingots 26 in place while the magazine 102 is indexed. The upper end ofeach arm 60 engages the evaporating ingot 26, which facilitates feedingthe ingot 26 in an upward direction with an elevator 61 without allowingthe clamping arm 60 to slide downward toward a horizontal position,which was determined to cause jamming of the feed mechanism. Accordingto the invention, each magazine 102 sequentially aligns the next ingot26 with the lower end of the evaporating ingot 26 within the crucible56, and the elevator 61 feeds the next ingot 26 into the coating chamber12 behind the evaporating ingot 26, with no or minimal interruption ofthe deposition of the ceramic material on the parts 20.

The viewport 48 noted in reference to FIGS. 3 through 5 is shown ingreater detail in FIG. 13. The viewport 48 is configured to permit theoperator of the apparatus 10 to observe the coating operation, includingthe parts 20 being coated, the pools of molten ceramic, the reflectors80 around the crucibles 56, and the manipulators 77 used to move thereflectors 80. As shown, the viewport 48 is generally an enclosure thatincludes a fluid-cooled aperture plate 106 with an optional window 108formed of sapphire in order to withstand the high temperatures (roughly800° C. or more) in proximity to the coating process. A shielding gas isshown as being directed toward the aperture plate 106 through a port 110for the purpose of minimizing coating deposition on the window 108 orequipment behind the aperture plate 106. Within the viewport 48, arotating stroboscopic drum 112 serves to minimize exposure of a viewingwindow 114 to radiant heat, light and other radiation from the coatingchamber 12. In accordance with known practice, the drum 112 has slots116 through its wall and rotates at a high rate to eliminate visualflicker to the eye of the observer. The window 114 is preferably amultiple-pane of quartz glass, lead glass and/or colored glass. Thequartz glass provides physical strength, the lead glass providesprotection from x-rays, and the colored glass is useful to reduce lightintensity. The viewport 48 further includes a magnetic particle sealthat provides a high-temperature vacuum seal for the stroboscopic drum.Another preferred feature is that the viewport 48 provides astereoscopic view of the interior of the coating chamber 12, by whichone or more operators can simultaneously observe the coating chamberwhile retaining depth perception.

Shown in FIG. 14 is a preferred control panel 118 for controlling andmonitoring the EBPVD apparatus 10 of this invention. The control panel118 is shown as including a schematic of the apparatus 10 and itscomponents, including indicia 120 for individual components (e.g., thecoating chamber 12). Also shown are visual indicators 122 locatedadjacent the indicia 120 for indicating the operating status of thecomponents, and switches 124 to change the operation of thecorresponding components. The panel 118 is preferably surrounded bygauges for quantifying process parameters, such as pressures. With thepanel 118, information regarding the operating status of the EBPVDapparatus 10 can be quickly and accurately noted to allow the operatorto make any appropriate adjustments to the apparatus 10 and the coatingprocess.

In operation, the apparatus 10 of this invention may initially appear asshown in FIGS. 1 and 2. As discussed previously, the parts 20 to becoated are loaded onto the rakes 22 within the loading chambers 16 and18. The parts 20 may be formed of any suitable material, such as anickel-base or cobalt-base superalloy if the parts 20 are blades of agas turbine engine. In the case of gas turbine engine blades, prior tocoating with the apparatus 10, the surfaces of the parts will typicallybe provided with a bond coat of known composition as discussedpreviously. Also prior to depositing the ceramic TBC, the surface of thebond coat is preferably grit blasted to clean the bond coat surface andproduce an optimum surface finish required for depositing columnar EBPVDceramic coatings. Also prior to depositing the ceramic coating, analumina scale is preferably formed on the bond coat at an elevatedtemperature to promote adhesion of the coating. The alumina scale, oftenreferred to as a thermally grown oxide or TGO, develops from oxidationof the aluminum-containing bond coat either through exposure to elevatedtemperatures prior to or during deposition of the ceramic coating, or byway of a high temperature treatment specifically performed for thispurpose. According to this invention, the parts 20 are preferablypreheated to about 1100° C. in an argon atmosphere. When not being usedto preheat parts 20, the preheat chamber 14 is preferably maintained atabout 600° C. to minimize the temperature range to which the chamber 14is subjected during a campaign.

After preheating within the preheat chamber 14, the rakes 22 are furtherextended into the coating chamber 12. As previously noted, the apparatus10 of this invention is particularly configured to deposit a ceramiccoating under the elevated pressure conditions taught by Rigney et al.Prior to initiating the coating process, a quick vacuum check ispreferably performed to track the pumpdown rate and pressure achievedwithin each of the coating, preheat and loading chambers 12, 14, 16 and18 during a set time period. Doing so serves to determine the vacuumintegrity of the apparatus 10, which was previously performed with priorart EBPVD operations through an oxidation test performed on sacrificialspecimens. The chambers 12, 14, 16 and 18 are evacuated with themechanical pumps 31 from atmospheric pressure, and then a blowercommenced when pressures drop to around 20 mbar. The cryogenic pump 32is preferably started when a pressure of about 5×10⁻¹ mbar is reached.Thereafter, the diffusion pumps 32 and 34 are started for the coatingand preheat chambers 12 and 14 when a pressure of about 5×10⁻² mbar isreached. Suitable process pressures within the loading and preheatchambers 14, 16 and 18 are about 10⁻³ to 10⁻¹ mbar, with suitablecoating pressures being about 10⁻² to about 5×10⁻² mbar within thecoating region defined by the hood 52. A dual-element ion gauge 55provided with a manual shutoff valve 57 is preferably used to measurethe vacuum pressure within the coating chamber 12. By using a gauge 55with independently operable elements, either element can be selected foruse without interrupting the coating operation. Alternatively, two iongauges separated by a valve could be provided, so that either gaugecould be used or switched without interrupting the coating operation.

In a preferred aspect of this invention, the cryogenic pump 32 ispreferably started prior to the diffusion pump 34, contrary to priorpractice in which both pumps 32 and 34 were typically started at thesame time to minimize ice buildup on the cryogenic pump 32. Starting thecryogenic pump 32 before the diffusion pump 34 has been found tosignificantly reduce the amount of time required to attain the coatingchamber pressures desired for this invention. While starting thecryogenic pump 32 prior to the diffusion pump 34 promotes ice buildup onthe cryogenic pump 32, this ice can be removed at the end of a coatingcampaign or any other convenient time.

During the coating operation, the electron beams 28 are focused on theingots 26, thereby forming the molten pools of ceramic and vapors thatdeposit on the parts 20. While various coating materials could be used,a preferred ceramic material for TBC (and therefore the ingots 26) iszirconia (ZrO₂) partially or fully stabilized by yttria (e.g., 3%-20%,preferably 4%-8% Y₂O₃), though yttria stabilized with magnesia, ceria,calcia, scandia or other oxides could be used. The coating operationcontinues until the desired thickness for the coating on the parts 20 isobtained, after which the parts 20 are transferred through the preheatchamber 14 to the loading chamber 16, after which the loading chamber 16is vented to atmosphere. The vents are preferably at least 30 mm indiameter in order to increase the venting rate, but generally less thanabout 60 mm in diameter to avoid disturbing dust and other possiblecontaminants within the chambers 12, 14, 16 and 18. For this reason, itmay be desirable to initially vent with a smaller diameter valve,followed by a larger diameter valve.

While our invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Accordingly, the scope of our invention is to belimited only by the following claims.

1. A method of operating an electron beam physical vapor depositioncoating apparatus, the method comprising the steps of: mounting anarticle on a support within a loading chamber that is adjacent a preheatchamber; establishing an absolute pressure of between 10⁻³ mbar and 10⁻¹mbar within the loading chamber and the preheat chamber; moving thearticle into the preheat chamber; establishing an elevated temperatureand an absolute pressure of between about 10⁻³ mbar and 5×10⁻² mbarwithin a coating zone defined by a condensate hood located within acoating chamber adjacent the preheat chamber, the absolute pressurewithin the coating zone being higher than a pressure within a region ofthe coating chamber outside the condensate hood; moving the article intothe coating zone within the condensate hood, coating the condensate hoodhaving a reflective member located within the coating chamber such thatthe article is between the reflective member and a coating materiallocated within the coating chamber, the reflective member being in afirst position relative to the molten coating material; operating anelectron beam gun to project an electron beam onto the ceramic materialso as to form a molten pool of the coating material, form vapors of thecoating material within the coating zone and deposit the vapors on thearticle, the article being subject to heat radiated by the molten pooland to radiative heat emitted by the molten pool and reflected backtoward the article by the reflective member; and as the temperaturewithin the coating zone rises, moving the reflective member to a secondposition farther further from the molten pool of the coating material sothat the article is subject to less reflective heating by the reflectivemember, the reflective member operating with the condensate hood so thatthe absolute pressure within the coating zone remains higher than thepressure within the region of the coating chamber outside the condensatehood.
 2. A method according to claim 1, wherein the absolute pressure isestablished within the coating zone by first operating a mechanicalpump, followed by a cryogenic pump and then a diffusion pump, thediffusion pump comprising a throttle valve operable between an openposition, a closed position, and at least one intermediate positiontherebetween, wherein the throttle valve is positioned at the openposition to evacuate the coating chamber and establish the absolutepressure within the coating zone, and then the throttle valve ispositioned at the at least one intermediate position during thedepositing step to maintain at the absolute pressure within the coatingzone.
 3. A method according to claim 1, wherein the molten pool of thecoating material is contained by a fluid-cooled crucible, wherein theelectron beam is projected onto the coating material to define anelectron beam pattern on the coating material, the apparatus furthercomprising means for projecting a separate electron beam pattern on thecrucible for evaporating molten droplets of the molten coating materialon the crucible, the separate electron beam pattern having a higherpattern intensity than the electron beam pattern on the coatingmaterial.
 4. A method according to claim 3, wherein the separateelectron beam pattern is selectively projected while viewing the moltendroplets of the molten coating material through a viewport.
 5. A methodaccording to claim 1, wherein the coating material is in the form of aningot supported by a rotatable magazine beneath the coating chamber, themagazine supporting a plurality of ingots of the coating material, themethod further comprising the step of the magazine incrementallyaligning each of the ingots with an aperture to the coating chamber forsequentially feeding the ingots into the coating chamber.
 6. A methodaccording to claim 1, wherein the absolute pressure established withinthe coating zone chamber is between about 10⁻² mbar and 5×10⁻² mbar. 7.A method according to claim 1, further comprising the steps of: placinga second reflective member adjacent the molten pool of the coatingmaterial such that the article is also subject to reflective heatingfrom the second reflective member; and moving the second reflectivemember toward and away from the molten pool so as to increase anddecrease, respectively, the reflective heating from the secondreflective member to the article.
 8. A method according to claim 7,wherein the second reflective member is a ceramic material or as coatedwith a ceramic material.
 9. A method according to claim 7, wherein theplacing step comprises placing a plurality of the second reflectivemember adjacent the molten pool of the coating material, and the movingstep comprises selectively and individually moving at least one of thesecond reflective members toward and away from the molten pool.
 10. Amethod according to claim 1 wherein, while the reflector member is inthe second position, a shroud surrounds the reflective member andinhibits gas flow between the condensate hood and the reflector memberto reduce pressure loss from the coating zone.
 11. A method of operatingan electron beam physical vapor deposition coating apparatus thatcomprises a coating chamber, a condensate hood that defines a coatingzone within the coating chamber, a coating material within the coatingchamber, and at least one electron beam gun for projecting an electronbeam onto the coating material, the method comprising the steps of:establishing an elevated temperature and an absolute pressure within thecoating zone; placing an article in the coating zone, the condensatehood comprising a reflective member such that the article is between thereflective member and the coating material, the reflective member beingin a first position relative to the coating material; operating theelectron beam gun to project the electron beam onto the ceramic materialso as to form a molten pool of the coating material, form vapors of thecoating material within the coating zone, and deposit the vapors on thearticle, the article being subject to heat radiated by the molten pooland to radiative heat emitted by the molten pool and reflected backtoward the article by the reflective member; and as the temperaturewithin the coating zone rises, moving the reflective member to a secondposition farther from the molten pool so that the article is subject toless reflective heating by the reflective member.
 12. A method accordingto claim 11, further comprising the steps of: placing a secondreflective member adjacent the molten pool of the coating material suchthat the article is also subject to reflective hcalng from the secondreflective member, and moving the second reflective member toward andaway from the molten pool so as to increase and decrease, respectively,the reflective heating from the second reflective member to the article.13. A method according to claim 12, wherein the second reflective memberis a ceramic material or is coated with a ceramic material.
 14. A methodaccording to claim 12, wherein the placing step comprises placing aplurality of the second reflective member adjacent the molten pool, andthe moving step comprises selectively and individually moving at leastone of the second reflective members toward and away from the moltenpool.
 15. A method according to claim 11, wherein the absolute pressurewithin the coating zone is higher than a pressure within a region of thecoating chamber outside the condensate hood, and when in the first andsecond positions thereof the reflective member operates in combinationwith the condensate hood so that the absolute pressure within thecoating zone remains higher than the pressure within the region of thecoating chamber outside the condensate hood.
 16. A method according toclaim 15, wherein, while the reflector member is in the second position,a shroud surrounds the reflective member and inhibits gas flow betweenthe condensate hood and the reflector member to reduce pressure lossfrom the coating zone.
 17. A method according to claim 11, wherein theabsolute pressure established within the costing zone is between about10⁻² mbar and 5×10⁻² mbar.